Hybrid optical recording discs are discs having a read-only memory (ROM) area and a recordable area for recording or writing data which are usually generated by a computer user and which are recorded on the disc by a recorder or writer controlled by a computer. Such a disc has a substrate which can be formed by injection molding against a master plate so that the mastered substrate will have a continuous spiral track extending from an inner edge to an outer edge of the substrate. The spiral tack is usually a groove which provides data channels on the disc and also provides for tracking of the disc while reading or recording data. The groove is frequency-modulated in a direction normal to the groove and is, therefore, referred to as a wobbled groove or a wobble groove. In the ROM area of a hybrid optical disc the groove is further modulated in the form of depressions which correspond to disc addressing data and to disc program data. The mastered substrate is then coated with a recording layer which can include an organic dye selected to absorb radiation in the recordable area. Upon coating the recording layer, a reflective layer is formed over the entire recording layer. A protective layer, usually of a polymer organic material, is formed over the reflective layer.
The tracks or grooves of a hybrid optical recording disc, the degree of modulation of the groove, as well as the arrangement of addressing and program data is usually provided in accordance with Orange Book specifications. “Orange Book” is a specification published by Philips Corporation and Sony Corporation which defines key properties of recordable compact disc media and recording performance.
The aforementioned master plate is used to produce numerous plastic disc substrates by embossing or injection molding techniques. Such a master plate is also referred to as a stamper. The stamper, in turn, is produced by metal plating techniques whereby a photoresist master having the spiral groove and the data depressions is plated with a metal. The metal layer is then separated from the photoresist master and constitutes the stamper which replicates the features of the photoresist master in an inverted orientation, i.e. a groove in the photoresist master will be a projection in the stamper. The photoresist master disc comprises a photoresist layer formed over a substrate which is usually a glass substrate. Accordingly, the photoresist master is also referred to in the art as a glass master.
Depending on the configuration and on the intended application of an optical disc, particular challenges and problems need to be addressed to meet and to retain specifications in accordance with standards such as, for example, defined in the aforementioned Orange Book or in a Red Book, also published by =Philips Corporation and Sony Corporation. For example, Horie et al., U.S. Pat. No. 5,862,123 disclose optical phase-change material formed on a substrate which has a wobbled spiral groove on a substrate. Particular relationships are selected between a groove width, a laser beam diameter, and a wobble amplitude to prevent distortion of the groove caused by repeated over-writing operations. In FIG. 10 of U.S. Pat. No. 5,862,123 to Horie et al., a block diagram of a laser beam recording apparatus is shown for recording grooves and data on a photoresist layer formed over a glass substrate, i.e. for recording a “glass master” having the selected particular relationships.
Udagawa, U.S. Pat. No. 5,737,289 discloses a data recording apparatus providing different plural recording laser powers within respective subcode frames constituting sub-partitions of an optical disc so as to determine an optimum laser drive power. Also disclosed are laser drive circuit and control circuitry.
Wilkinson et al., U.S. Pat. No. 5,297,129 disclose a method and apparatus for shaping the waveform of laser pulses to achieve improved characteristics (leading and trailing edges) of surface effects recorded on an optical disc. An optical modulator is used to modulate the intensity of a laser beam either above or below a threshold level to either produce surface effects on a moving recording medium, or to be incapable of producing surface effects on the moving recording medium when the laser beam intensity is below a threshold level.
The fabrication of a photoresist master disc dedicated to forming a hybrid optical recording disc poses significant technological challenges which differ from the challenges encountered by the above cited references. Firstly, conventional laser beam recording systems for recording a photoresist master dedicated for a conventional CD-ROM do not have the ability to record a photoresist master in a multi-session format required of a hybrid optical recording disc. Nor do such conventional CD-ROM laser beam recording systems provide a capability to record frequency-modulated or wobbled grooves. For example, in a hybrid optical disc, the ROM area includes a first session which includes a ROM lead-in area, a ROM program area, and a ROM lead-out area. The recordable area of the hybrid optical disc constitutes a second session comprising a lead-in area, a recordable program area, and a lead-out area. Since the hybrid optical recording disc has a recordable area, the photoresist master for such a disc must be recorded so as to simulate a CD writer and thus write the first session and leaving it open for appending, i.e. for subsequent writing in the recordable area of the hybrid optical disc, the writing to be performed by a computer user. It is also required to record in such a photoresist master various codes and addressing data which allow a writer to recognize the hybrid optical disc as being writable.
Furthermore, a photoresist master for a hybrid optical disc must meet particular specifications within a transition area between the lead-out area of a recorded first session and a lead-in area of the recordable second session.
In particular, the transition between the ROM area and the recordable area has a specification of a 26EFM frame interval. An EFM generator is used to modulate the intensity of a laser beam so as to record a continuous spiral groove and data in portions of the groove in the form of depressions. A frame has a typical duration of about 130 microseconds. Conventional laser beam recording systems designed for recording photoresist masters for conventional CD-ROMs rely on decoding a subcode in the EFM data stream to change or to modulate the recording laser beam via a coded channel such as a RS232 channel. However, the response of a typical RS232 channel is in a range of milliseconds, a response time which is too long for meeting the 26EFM frame interval requirement accurately and consistently.
Thus, existing conventional laser beam recording systems for recording photoresist masters of conventional CD-ROMs have to be modified in order to have a performance which meets the requirements of recording a photoresist master for a multi-session hybrid optical recording disc.